
From the computational point of view, the molecule as a polarizable point dipole
when modelling the optical properties of molecules close to metal nanoparticles.


physically, the fluorescence intensity follows the field enhancement[], and
it related closly with SERS.

Q: What is the difference between radiative and nonradiative processes?

In the rate-equation above, it is assumed that decay of the number of
excited states N only occurs under emission of light. In this case one
speaks of full radiative decay and this means that the quantum efficiency
is 100 percents. Besides radiative decay, which occurs under the emission
of light, there is a second decay mechanism; nonradiative decay. To
determine the total decay rate Γtot, radiative and nonradiative rates
should be summed:

Γtot = Γrad + Γnrad 

where Γtot is the total decay rate, Γrad is the radiative decay rate and
Γnrad the nonradiative decay rate. The quantum efficiency (QE) is defined
as the fraction of emission processes in which emission of light is
involved:

Qe = Γrad/(Γrad + Γnrad)

In nonradiative relaxation, the energy is released as phonons, more
commonly known as heat. Nonradiative relaxation occurs when the energy
difference between the levels is very small, and these typically occur on a
much faster time scale than radiative transitions. For many materials (for
instance, semiconductors), electrons move quickly from a high energy level
to a meta-stable level via small nonradiative transitions and then make the
final move down to the bottom level via an optical or radiative
transition. This final transition is the transition over the bandgap in
semiconductors. Large nonradiative transitions do not occur frequently
because the crystal structure generally can not support large vibrations
without destroying bonds (which generally doesn't happen for
relaxation). Meta-stable states form a very important feature that is
exploited in the construction of lasers. Specifically, since electrons
decay slowly from them, they can be piled up in this state without too much
loss and then stimulated emission can be used to boost an optical signal.
